The best things you will find in Agra are petha, parantha, Mughlai, Bhalla, Dalmoth, Tandoori Chicken, Jalebi and Shawarma to name a few. I have been their twice and trust me you do not want to miss on these all.
Panchi Petha, Dal moth and Patisa from Hathras, Ram babu for parathas trans Yamuna, Chat at Sadar bazzar, Bedai and aloo sbazi for breakfast at Pratapura.